Polyurethane flexible foams of high elasticity ("HR foams") are prepared from polyols containing predominantly primary hydroxyl groups, polyisocyanates, water, organic blowing agents, activators, and crosslinking agents. Alkanolamines such as diethanolamine and triethanolamine have proved to be satisfactory crosslinking agents, but amine-free low molecular weight polyhydroxyl compounds such as glycerol, pentaerythritol, sorbitol, mannitol, and dulcitol are also suitable. These products are described, for example, in German Offenlegungsschrift 2,728,031 and U.S. Pat. No. 4,883,825, and their use for the preparation of HR foams is considered state of the art. U.S. Pat. No. 4,883,825 also describes the use of certain combinations of diethanolamine and polyhydroxy components as crosslinking agents for HR molded foams of low gross density.
Block foams of low compression resistance have previously been prepared with the aid of certain proportions of fluorochlorohydrocarbon blowing agents, mainly fluorotrichloromethanes, but this class of compounds is considered not to be ecologically harmless. It was, therefore, an object of the present invention to produce flexible HR block foams over a wide range of gross densities without the aid of fluorochlorohydrocarbons. It was surprisingly found that when preparing flexible, highly elastic block foams, the method of dosing the crosslinking agents is extremely important if stable foam having the desired profile of properties are to be obtained.